Lightning arrester



July 11, 1939- 'R. H. EARLE ET AL 2,165,964

LIGHTNING ARRESTER Filed April 4, 1956 3 Sheets-Sheet l.

Hllvllllll 29 @ven/fori- R. H. EARLE El' AL LIGHTNING ARBSTER July 11,193.A

3 Sheets-Sheet 2 F'iled April 4, 1936 July 11, 1939.

v R. H. EARLE El' AL LIGHTNING ARRESTER Filed April 4, 1936 3Sheets-Sheet 5 Y Y v 47@ I I I I I I I I I I I I I I I I Patented July11, 1939 UNITED STATES PATENT OFFICE IJGHTNING ARRESTEB Ralph H. Earle,Wauwatosa, and Alwin G. Steinmayer, Milwaukee, Wis., assignors to LineMateria] Company, South Milwaukee, Wis., a cor poration o! DelawareContinuation of application Serial No. 716,244, March 19, 1934. Thisapplication April 4, 1936,

Serial No. 72,734

1 Claim.

' arrester will immediately relieve the system of any abnormal voltageby providing a conducting path to ground through which current can ilowbut which will not permit the flow of power current under normaloperating conditions at normal line voltage. Ihat is, during normaloperating conditions the arresteroifers a substantially .inilniteresistance to the. flow of power current at the system voltage but onthe occurrence of an abnormal voltage caused by lightning or switchingdisturbances, the resistance oi' the arrester becomes such that itprovides a path to ground through which the surge current can ow untilthe voltage causing it has been dissipated, at which time, however,power current is not permitted to ow.

A typical lightning arrester for distribution systems comprises a sparkgap assembly arranged to be connected in series circuit relation withvalve material that oilers a comparatively high resistance to the .ilowoi' current. An arrester, comprising the series connected spark gapassembly and valve material, is connected between each of thetransmission conductors and ground. The valve material may comprisesilicon carbide in various forms, either granular or in blocks, or-

it may comprise lead peroxide pellets which, upon heating. become coatedat their contact points with litharge. Various other types of valvematerial may also be used.

It has been customary in the construction of lightning arresters in the4prior art to provide them with an opaque housing of ceramic materialsuch as porcelain around the spark gap assembly and valve material. Thelhousing serves as a frame for supporting the arrester element as aunitary structure and permits mounting of the arrester on a pole ortransmission tower for connection to a transmission conductor.

On ilow of surge current, due either to a lightning stroke or to aswitching disturbance, a conducting path is momentarily establishedthrough the arrester which, in the ideal arrester, as statedhereinbefore, should be disrupted on termination of ilow of surgecurrent. l:in this manner the arrester simulates the action of a valvewhich is opened on occurrence of an. abnormal voltage on the system topermit the ow of surge current to ground but which is automaticallyclosed as vsoon as the excess voltage is dissipated. If the valve doesnot close or the conducting path is not disrupted, then power current ofnormal line `frequency may flow, causing an undesirable leak or powerloss and also destroying the eilectiveness of the arrester forprotecting the system against flow of power current after a succeedingabnormal voltage condition.

'I'heflow of surge current through the arrester due to abnormal voltagecaused by a lightning or a switching disturbance takes place only duringan innitesimally short time. Ordinarily, the surge current will ilowonly during a i'ew microseconds. However, the amount of current whichows may be relatively great and in many cases it is enough to destroythe arrester entirely or to crack the porcelain housing. In such casethecustomary casual inspection will reveal the failure o1' the arrester andit may be replaced with a new one.

In many instances, however, the ow of surge current is not enough tototally or even partially destroy the arrester or its housing, but itmay be sumcient to fuse some oi the particles, forming the valvematerial, into a conducting path of comparatively low resistance to flowof power current. After the ow of surge current has ceased, theresistance of the arrester is not restored toits normal or substantiallyinfinite resistance to ilow of current at power frequency, and a slightdischarge or flow oi' power current takes place through it. When anarrester is operating under these conditions, it should be replaced,since it no longer is capable of interrupting the ow of power currentafter a surge breaks down the gap. 'I'he next surge may cause thearrester to become fully grounded, thereby possibly causing a shortcircuit on the system, necessitating the operation of circuit breakersor fuses to clear the fault.

The'continued now of power current through the path rendered conductingby the surge current and across the spark gaps, causes the arrester tofunction somewhat in the manner of a spark gap radio transmitter. lineacts in this case as the antenna of a radio The transmission if ltransmitter. Although the radiating powerV of -such a transmitter may becomparatively slight,

still considerable interference with radio reception results,particularly in those instances where radio receivers are operated inthe vicinity of the power line. The radio interference may besuiliciently severe to totally prevent the reception of any broadcastprogram or radio communication.

When arresters of the prior art are employed using opaque housings oprocelain or the like, it is not possible to determine whether or notthey have failed to this extent Without a careful examination andtesting of each arrester. This means that a periodic examination of thearresters must be made and a test conducted to determine theirresistance or a succeeding surge must be awaited to completely destroythe arrester or cause it to ground the system. Even when the arresterhas been grounded, unless it is fractured in some vmanner by the flowtherethrough of surge or power current, it may be difficult to ascertainthe exact location of the fault. Under such conditions it may benecessary to disconnect a large number of arresters which are in properoperating condition in order to isolate the particular arrester that hasfailed.

'I'he object of our invention, generally stated, is

.to provide a surge arrester for transmission lines of commercialfrequency which shall be simple and efficient in operation and which maybe readily and economically manufactured, installed and4 inspected.

The principal object of our invention is to provide for visuallyindicating that a surge arrester on a power line has failed.

An important object of our invention is to provide for visuallyindicating the flow of power current through a path in a lightningarrester rendered conducting by a lightning discharge.

Still another object of our invention is to provide for renderingvisible from any point the light emitted on flow of power current from apath in a lightning arrester rendered conducting by a lightningdischarge.

A more specic object of our invention is to provide a lightning arrestercomprising spark gap and'valve material assemblies with a housing oflight transmitting .materiaL whereby the light emitted on ow of powercurrent through a path in the valve material rendered conducting by alightning discharge, will be visible.

Other objects of our invention will in part be obvious' and in partappear hereinafter.

Our invention accordingly is disclosed in the embodiments hereof shownin the accompanying drawings and it comprises the features ofconstruction, combination of elements and arrangement of parts whichwill be exemplified in the constructions hereinafter set forth, and thescope of the application of which will be-indicated in the appendedclaim.

For a more completev understanding of the# nature and scope of ourinvention reference may be had to the following detailed description,taken in connection with the accompanying drawings, in which- Figures 1,2 and 3 illustrate, diagrammatically, different circuit connections forlightning arresters assapplied to commercial power distribution systems;

Figures 4 and 5 show curves which illustrate certain operatingcharacteristics of a power distribution system on application thereto oflightning or switching disturbances;

aieaces Figure 6 is a view, in side elevation-of a lightning arresterconstructed in accordance with our invention;

Figure 'l is a sectional view showing the interior details ofconstruction of our novel lightning arrester;

Figure 8 is a diagrammatic view showing different operating conditionsof a lightning arrester on iiow therethrough of surge current; and

Figures 9, 10 and li are views showing different types of faults whichmay occur in lightning arresters as a result of the application to atransmission systern of lightning or switching surges.

As a result of a careful study of the results of lightning and switchingdisturbances on a power system having connected thereto lightningarresters for draining off the abnormal voltages, we have discoveredthat the discharge takes place in the arrester, in most instances, alongthe inner periphery of the housing surrounding the spark gap assemblyand valve material. That is, on the occurrence of a comparatively lightsurge,

the discharge takes place through the arrester` current in each of whichis suftlcient to damage f" the arrester but is not great enough todestroy it or even to crack the housing so as to entirely clear the lineof the arrester. We have observed that it is this type of dischargewhich takes piace generally along the inner wall or periphery of the fhousing surrounding the spark gap assembly and valve material. 'I'hisdischarge is suicient, in many instances, to fuse together a number ofthe particles forming the valve material, so that a path ofcomparatively lowresistance is formed in lthe arrester, thereby loweringits resistance and permitting the ow therethrough of a slight amount ofpower current at power frequency' from the transmission line.

Since the resistance of the arrester under these conditions stillremains at a comparatively high value, only a small amount of powercurrent will iiow therethrough. However, this amount of currentconstitutes a leak from the power system and, in owing between theelectrodes of thc spark gap assembly, generates oscillations of radiofrequency which tend to interfere with radio reception in the vicinityof the power line. A more serious objection, however, is that the ei-Iectiveness of the arrester for protecting the transmission line fromoutages caused by the ow of power current after static surges passthrough the arrester, is largely destroyed. Therefore, an arrester whichis operating under these conditions should at once be removed from thesystem and replaced with a new arrester in proper operating condition.

While\it iscomparatively easy to inspect a transmission line and notearresters that have cycle generator.

the fact that it has been the practice in the past to-house the sparkgap and valve material assemblies in an opaque housing of porcelain orthe like through which it is, of course, impossible to see from theexterior, and likewise through which it is impossible for light rays 1obe transmitted, caused by current flowing along paths in the arresterthat have been rendered conducting by lightning or switching surges.

In order to provide for readily inspecting lightning arresters asinstalled on a transmission line without making a detailed examinationand test,

we have dispensed with the customary opaque' porcelain housing and havesubstituted therefor a housing of relatively thick light-transmittingmaterial such as glass, and preferably of a heat-resisting type whichmay be purchased in the open market under the name of Pyrex. Since theflow of power current along a path rendered conducting by a lightningdischarge generally takes place along the inner surface of the glasshousing, light rays emanating therefrom are readily visible through theglass housing and, as a result, it is a simple matter to determine byinspection, even when the observer is some distance away from thearrester, that it has failed and that it should be replaced. Moreover,the housing of glass is made comparatively thick, so that light raysemanating from along a path at one side of the arrester will be visiblefrom any point thereabout, due to the reiiecting characteristics of theglass wall of the housing.

In order to illustrate in more detail thevarious applications of ournovel lightning arrester to power systems of 60 cycles or less, certaincircuits are illustrated in the drawings. These circuits are typical ofthel distribution circuits which it is the present day practice to usefor distributing electric power for domestic and industrial purposes.

Referring now particularly to Figure 1 of the drawings, it will beobserved that a 3-phase transformer, shown generally at I5, is provided,having a delta connected primary winding I6 which may be connected forenergization to a source II of alternating current, such as a 60 Thetransformer I5 is provided with a star connected secondary winding I8,the terminals of which may be connected through disconnecting switchesI9 to energize transmission line conductors A, B and C. 'I'he neutralpoint 20 lof the secondary winding I8 is grounded at 2l, as illustrated.Since each phase of the secondary winding I8 is arranged to have 2300volts impressed thereacross, a voltage of 4000 volts appears betweeneach ofthe conductors A, B and C, as illustrated.

In order to provide for connecting the high voltage conductors A, B andC to a secondary distribution system for domestic or industrial use, theprimary winding 22 of a distribution transformer, shown generally at 23,is connected across the conductors B and C. The secondary winding 24 ofthe distribution transformer 23 is provided with a mid-tap so that athree-wire volt circuit is provided, as is customary-for residentialuse.

In order to protect the distribution transformer 23 from surge 'currentdue to lightning and switching disturbances, lightning arresters, showngenerally at 25, are provided. Each of the arresters 25 comprises aseries of spark gaps 26 connected to a resistor 21 having acomparatively high resistance such as valve material comprising siliconcarbide or the like. The arresters 25 may be grounded at 28 and 25.

' If a lightning or switching surge occurs on the conductor C it willpass through the arrester 25 to ground at 28, breaking down the sparkgaps 26 and causing current to flow in an amount corresponding to thevoltage accompanying the disturbance. It will be noted that in thisinstance a line voltage of 2300 volts is applied across the arrester 25between the conductor C and ground 28, which is, of course, electricallyconnected to the 'ground 2| and the neutral point 20 of the secondarywinding I8. If the amount of surge current is such that the arrester 25is capable of handling it without failure or darnage thereto, thearrester will interrupt the follow current when the next zeropoint inthe wave of power current occurs and the system will be restored tonormal operating conditions.

As a modification of the circuit connections shown in Figure 1, thecircuit connections shown in Figure 2 may be used. It will be here notedthat a neutral or ground conductor G is provided which is connected tothe neutral point 20 of the secondary winding I8 of the transformer I5.This conductor G is carried along with the conductors A, B and C,forming the transmission line, and it is grounded at various points, 30and 3I, for example. In this instance the primary winding 22 of thedistribution transformer 23 is connected between the conductor C and theground or neutral conductor G and it has applied thereto 2300 volts. Ifa lightning or switching surge strikes the conductor C the dischargethrough the arrester 25 to ground 28 will take place as describedhereinbefore in connection with Figure 1. However, if the surge shouldstrike the ground or neutral'wire G, it may pass to ground through oneof the ground connections" at 30 or 3|, or if there is no groundconnection close enough, the surge will travel along the neutral wire Guntil it comes to a lightning arrester which may be connected thereto,at which point y it will pass to ground. Since the conductor G is atground potential, there will be no follow current and, as a result, theonly damage that can possiblyv be done results from the ow of surgecurrentjand not from the flow offollow current.

Still another form of transmission circuit which may be employed isillustrated in Figure 3 of the drawings. As there shown, the transformerI5 is provided having its secondary winding I8 connected in delta to theconductors A, B and C. Each phase of the secondary winding I8 may haveapplied thereto, as indicated,-4800 volts and this voltage is applied tothe primary winding 22 of the distribution transformer 23 connectedacross the conductors B and C. In the event that a lightning orswitching surge appears on one or the other of the conductors B or C, itwill be dissipated through either of the arresters 25 to the groundconnections 28 or 23, respectively. Since the secondary winding I8 ofthe transformer I5 is not grounded, there will be no follow current andthe only damage that can occur, providing the remaining conductor A ofthe power line is ungrounded, will be due to the flow of surge current.fIt often happens, however, that an accidental ground, as at 32, occurson one of the conductors, (here shown, the conductor A), and then thefull line voltage of 4800 volts will be effective to continue the ow ofpower current along the path whichha-s been rendered vconducting by theflow of surge current due to the occurrence of the abnormal or the flowof power current over the path which excessive voltage resulting fromthe lightning or switching disturbance.

From a consideration of the foregoing description of varioustransmission circuits which are now in use, it will be observed that theow of power current constitutes an important factor in the application,construction and operation of lightning arresters. If only the surgecurrent had to be considered, then the problem would be considerablysimplified. However, the path rendered conducting by the ow of surgecurrent, either as a continuous conducting path through contiguousparticles, or through an ionized atmosphere, instantly offers a path forthe flow of power current due to the ever present line voltage whichtends to maintain this current iiow. It will, therefore, be clear thatthe flow of surge current resulting from lightning or switchingdisturbances, prepares the Way for the ow of power current that may ormay not take place, depending upon the amount of current in the surge,the time of its occurrence, and the conditions created in the arresteras a result of its iiow therethrough.

In Figure 4 of the dr-awings, units of amperes o f current flow, 'duringa lightning discharge, are plotted as ordinates, while units of time, inmicroseconds, are plotted as abscissae, for the curve 36. The curve 36illustrates a typical relationship between the amperes of surge currentand the time of duration of the surge, which, as illustrated, may be ofthe order of 20 microseconds. The curve 31, shown in Figure 5 of thedrawings, is plotted with units of volts as ordinatesand units of timein microseconds as abscissae, for one cycle of a (iO-cycle alternatingcurrent. It will be observed that each half cycle corresponds toapproximately 8333 microseconds and therefore that the time during whichthe lightning surge takes place, as indicated by the curve 36 in Figure4 of fthe drawings, when plotted to the/scale used for'the curve 31,would be only the width of a very thin line. If the lightning dischargetakes pla'ce at the time then the follow current, as represented by thebalance of the curve 31, to its nextzero point, will tend to flow for atime corresponding t'o the balance of the curve.31 before it passesthrough its next zero point, assuming unity power factor. If the surgeoccurs at the time y, then the time during which follow current tends toflow will be correspondingly decreased. If the surge should occur at thetime z, corresponding to the Zero point of the voltage wave 31, thenthere would no tendency for flow of follow current unless the surgecurrent was of such a magnitude that the arrester fails and thereforeconducts power current during the next half-'cycle'.

, It will `,then be observed that the likelihood of the ilow of powercurrent during at least one half-cycle is comparatively great and thatit is has been rendered conducting by the surge current that constitutesone of the principal factors in the operation of a lightning arrester.If the -surge would always occur at a time corresponding to the zeropoint of the voltage wave 31 or at the time z, then the damage to thearrester caused by the flow of follow current would ordinarily notoccur. However, such is not the case under normal operating conditions,and there is always the' possibility that a certain amount o'f followcurrnt will ow for a large portion of a half cycle at least, duringwhich time a suiiiciently low resistance path may be created andmaintained` in the arrester so that during the next succeeding halfcycle the power current will continue to ow, although not in an amountwhich would be suilicient to cause destruction of the arrester. It is togive an indication of the flow of such current that our invention isparticularly useful and has provided a highly satisfactory means wherebythe flow of such current can be readily detected with a minimum ofinspection.

Referring now particularly to Figure 6 of the drawings, it will beobserved that a lightning arrester, shown generally at 4I, is thereillustrated', which is provided with a conductor 42 for connection toone of the power conductors of a transmission line and a terminal nut 43to whicha ground conductor may be connected by means of a terminalassembly 44. The arrester 4I is provided with a housing 45 which isformed preferably of a heat resisting glass that may be purchased in theopen market under the name of Pyrex, 'I'he outer surface of the glasshousing 45 is provided with beads 46 extending entirely or rester 4l aremore clearly shown in Figure 7 of the drawings. As there-illustrated, aspark gap assembly, shown generally at 49, is provided which maycomprise four cylindrical spark gap electrodes mounted betweeninsulating posts 5I which are provided with caps 52, preferably ofbrass, at each end thereof. By means of a screw 53 and aleaf spring 54,the top spark gap electrode 50 is connected to an upper terminal cap 55,preferably formed of copper, to which the conductor 42 may be connectedas by soldering. The upper end of the glass housing 45 is provided witha circumferential bead 56 around which the outer periphery of vthe uppercap may be crimped. Thelcap 41 is secured on top of the arrester 4| bymeans of a suitable cement 51, such as asphalt, which completely coversthe upper cap 55 and provides a liquid-tight connection thereover.

'I'he spark gap assembly 49 is mounted on a brass disc 59 which issurrounded, as illustrated, by means of the inwardlyturned edge of ametal disc 60 that may be expanded into engagement with the inner wallof the housing 45. An upper electrode A6I is provided which may beconnected by means of a screw 62 to the bottom spark gap electrode 50.The upper electrode 6I may be circular inform and composed of brass. Theouter periphery 63 of the upper electrode 6| may be curved downwardly,if desired, to tend to concentrate the flow of surge current along theinner surface of the housing 45. While the outer periphery 63 of theupper electrode 6I has been i1- lustrated as being curved downwardly, itwill be understood that such construction is not essential, and thatsatisfactoryoperation will result if the upper electrode 6l comprisesmerely a flat disc 4with the curved outer periphery 63 omitted. We

for engaging a ground connection between it and the nut 43 which isthreaded on the outside of the boss 55 as illustrated. A gasket 58,formed preferably of rubber, is positioned underneath the lowerelectrode 64 to provide a seal. Additionalsealing means are provided inthe form of a gasket 59 formed preferably of cork and a. brass washer10, which are interposed between the undersurface of the housing 45 andthe top of the nut 43, as shown.

Interposed between the upper and lower electrodes 5l and 54 is a mass ofvalve material Il which may comprise silicon carbide or other suitablevalve material well known to those skilled in the art. Under normaloperating conditions the valve material 1| ofi'ers a comparatively highresistance to the iiow of current therethrough. However, on theapplication of high or abnormal voltages due to lightning or switchingsurges, the resistance of the valve material 1I is decreased and surgecurrent is permitted 4to iiow therethrough. It will be observed that themass of valve material Il is positioned in the bottom of the glasshousing 45 in close contact with the inner periphery of the side wallsthereof.

As illustrated more clearly in the diagrammatic representation of thelightning arrester 4I shown in Figure 8 of the drawings, the lightningdischarge in many instances tends to take place along the lines betweenthe terminal cap 55 and the upper electrode 6I. This is due to the factthat the voltage drop between the cylindrical spark gap electrodes 50 atthe line or upper end 1s greater than it is near the ground or lower endof the arrester, due to the electrostatic capacity of the cylindricalspark gap electrodes 50. That is, the voltage drop across the spark gapassembly constitutes the major portion of the voltage drop across thearrester in many instances and, as a result, the arrester may break downor fail bev tween the cap 55 and the upper electrode 5i.

An illustration of this type of failure is shown at 80 in Figure 9 ofthe drawings. As there indif cated, the arrester 4I has failed along theinner surface of the glass housing 45. -Due to the formation of aconducting path by the surge current, causing the failure shown at 80, acomparatively low resistance path for the ow of power current isprovided through the arrester and it therefore permits a slight leak ofpower current. Moreover, the effectiveness of the arrester is destroyedandl it should at once be removed. Since the housing 45 is composed of alight-transmitting material, the fault III, which emits light rays ofsurge current is not sufficient to cause the particles of valve material1| to be fused into a conducting path, then on termination of the flowof surge current the arrester will be restored to its normal condition.

A typical example of a failure caused by this type of discharge isillustrated at 8| in Figure 10 of the drawings. As there shownfa portionof the glass housing 45.hasv been broken away to more clearly illustratethe conducting path which has been formed by fusion of particles of thevalve materiall 1| into a conducting path by a lightning or switchingsurge which exceeded the capacity of the arrester 4l. Since theparticles of valve material ll are fused together into a conducting pathadjacent the inner-surface of the glass housing 45, through which aslight amount of power current can iiow from the conductor to ground,the iiow of power current along it causes the path to become heated tosuch an extent that light rays are emitted therefrom. This iiow ofcurrent constitutes a leak from the power line and destroys theeifectivenes's Vof the arrester 4I. Moreover, due to the flow of currentacross the spark gap assembly 49, the arrester 4l operates after thefashion of a spark gap radio transmitter, as set forth hereinbefore,thereby causing radio interference in the vicinity of the transmissionline. When an arrester has failed in this manner it should beimmediately removed. from the system.V l

Since the glass housing 45 is relatively thickand'we have discoveredthat it should be at least one-eighth inch or greater in thickness-thelight rays emanating from the path 8| are reiiected to such an extentthat the discharge is visible from vany point around the arresterhousing 45.

cause a partial fusion of the glass housing 45 with a portion of theparticles forming the valve material 1I. The path 82 thus renderedconducting by the iiow of surge current therethrough is capablev ofconducting a comparatively large amount of power current, therebydestroying the etfectiveness of the arrester 4l and possibly causingconsiderable' radio interference.

"The typical failures of lightning arresters illustrated in Figures 9,10 and 11 are those which have actually occurred and which we haveobserved. Any arrester that has failed to the extent indicated in any ofthese figures should be immediately removed from the system, since itseffectiveness as an arrester has been destroyed. However, it would notbe possible to detect failures of this type in arresters which areprovided with opaque porcelain housings, as has been the standardpractice in the prior art. It would be necessary either to make adetailed examination of each arrester to determine which one had failedto the extent indicated in these figures orv it would be necessary toawait the occurrence of a succeeding lightning or switching surge whichlmay be'suilicient to completely destroy or ground the arrester topermit a detection of the location` -the iiow of surge current has beenso great as to our invention is particularly applicable.

It will therefore be apparent that we have provided a lightning arresterconstruction of novel form which immediately provides a visualindication of a partial failure thereof which has not been suflicient tocompletely destroy the arrester. The slight leakage or flow of powercurrent is at once rendered visible and a cursory inspection will revealthe particular arrester which has failed and which should be removedfrom the system and replaced with a new arrester in proper operatingcondition. It is reiterated that it is the flow of power current along apath which has been rendered conducting by means of a lightningdischarge that causes the path to be visible from any point around thearrester through the use of the glass housing 55 having relatively thickwalls. Such visibility is, of course, impossible if a housing ofporcelain or other opaque material is used.

Since certain further changes may be made in the foregoing constructionsand diierent embodiments of the invention may be made without departingfrom the scope thereof, it is intended that all matters disclosed in theforegoing deaieaeoa scription or shown in the accompanying drawings,shall be interpreted as illustrative and not in a. limiting sense.

We claim as our invention:

In a lightning arrester for an alternating current power transmissionline operating at a power frequency of 60 cycles or lessg incombination, a spark gap assembly, a pair of spaced apart electrodes, amass of electric valve material interposed therebetween, said spark gapassembly being connected in series circuit relation with said electrodesand valve material and the combination being disposed to be connectedbetween said line and ground, and a glass housing having relativelythick walls surrounding said spark gap assembly and said valve materialwhich permits light rays emanating from the interior thereof to bevisible from substantially any point around the lightning arrester andwhich permits a visual inspection to determine the existence of anyvariations from the normal condition thereof.

RALPH H. EARLE. ALWIN G. STEINMAYER.

